1. Field of the Invention
The present invention relates to a structure of a passively mode-locked optical fiber laser, and more particularly, to a structure of a laser for increasing the repetition rate of passively mode-locked pulses.
2. Description of the Related Art
In general, an optical fiber laser uses a rare-earth-doped fiber as a gain medium. Since the rare-earth-doped fiber has a very wide gain bandwidth, the optical fiber laser is suitable for obtaining a very short optical pulse of which duration is below 10.sup.-12 second.
On the other hand, a mode locking technique is used to produce a high peak power an optical pulse, and is classified into active and passive mode lockings. The active mode locking technique uses the periodic loss of a resonator induced by a modulator, and the repetition rate of mode-locked pulse can be easily varied by adjusting the frequency of a signal supplied to a modulator. The time duration of an actively mode-locked pulse is usually longer than that of a passively mode-locked pulse.
The passive mode locking technique utilizes a saturable absorber of which transmission rate varies according to the intensity of input light, and can usually produce an optical pulse of which time duration is shorter than that by the active mode locking technique. However, the passive mode locking technique has a problem that the repetition rate of the output optical pulses is usually fixed to a cavity round trip frequency.
The conventional passive mode locking of an optical fiber laser thus described utilizes an intensity-dependent nonlinear effect of an optical fiber, and has been studied through two kinds of laser structure, one is a figure-8 laser which uses a nonlinear amplifying loop mirror (NALM), and the other is a ring laser which uses nonlinear polarization rotation.